It is frequently necessary to measure illuminance or luminance at a number of different locations within a relatively short time period. This applies to both outdoor measurements and indoor measurements in a room. For example, in order to characterize sky luminance distribution, it is desirable to be able to measure the luminance across the sky vault at approximately one hundred locations or directions within a relatively short time interval to insure that the luminance distribution has not changed appreciably during the measurement period. In order to study lighting quality within an architectural space or a scale model of the space, it will often be desirable to measure the luminance of interior room surfaces when the room is lit by electric lighting only, by day lighting only, or with a combination of electric light and day light. Data on luminance distribution within the room is essential in order to calculate the glare index or some other measure of glare or visual comfort.
In other applications it may be desirable to measure the illuminance at many different locations within a building or within a scale model of a building. Each of these applications and other similar photometric or radiometric measurement applications have several requirements in common. They require: (1) relatively high absolute accuracy; (2) high precision in the measurement between channels; (3) minimum calibration drift over time; (4) ability to add or modify input channels; (5) the ability to collect data rapidly from a large number of channels; (6) the ability to transfer the collected data to an accessible file or storage device; (7) the ability to accept luminance and illuminance inputs; and (8) instrumentation which is rugged, durable and requires a minimum of maintenance and care. Low power consumption is also preferable so that, in some applications, the device can be battery-operated over long periods of time.
A number of different approaches have been used for this type of multichannel photometric measurements. One approach is to record a luminance distribution photographically, and then process the photographic record, using a microdensitometer to extract the relevant data. This approach requires a delay between collection of the data and its analysis and also generally suffers from severe calibration problems, since the relationship between film preparation and film development processes may vary with time and with the film production batch. Any single photographic frame will normally be limited to a hemispherical field of view.
The photographic approach can be modified by replacing the film with a video system which allows luminance at each location to be measured and automatically filed as an analog or digital signal in an electronic data bank, optical disc or in magnetic storage. The usefulness of this approach is limited by problems with linearity over a wide dynamic range, calibration, limited field of view, and the cost and complexity of the system.
Another technique is the use of a scanning photometer or radiometer. In this approach, a photometric sensor is mounted on a platform that mechanically scans over the desired field of view. Such systems have traditionally been used for measurements of sky luminance distribution. The scanning approach may also utilize a fixed photometric sensor with a scanning mirror system to send the light to the photometer. In either case, the mechanical requirements for a precision scanning device tend to make such approaches costly. The mechanical operation may also set a minimum scan time which may increase the total required time interval for a full scan sequence.
Still another approach is to design a device around an array of sensors, each of which operates independently, but all of which feed data to the same central unit. This approach will tend to produce a bulky unit if many sensors are required and will certainly be expensive, given the cost of high quality photometric sensors. In addition, due to the large number of individual sensors used, calibration between sensors becomes an important problem.